A typical device for direct (one-step) conversion of chemical energy into electricity utilizes fuel and oxidizer as reagents. Both reagents may be in gas, liquid, or solid (including paste) forms.
Batteries are electrochemical devices that irreversibly consume the reagents while supplying current to an external circuit. Rechargeable batteries are devices that reversibly consume the reagents, such that the initial reagents may be restored by supplying a current to the device from an external source. The major limitation of all batteries is their limited capacity, usually expressed in Ampere-hours. Rechargeable batteries have a limited number of charge-discharge cycles and thus eventually fail.
The fuel cell is another type of electrochemical device for generating electricity. Fuel cells are characterized by having open anode and cathode reaction chambers. Fuels cells operate when fuel is supplied into the anode chamber and oxidizer is supplied into the cathode chamber. Fuel cells do not have such disadvantages as limited capacity and a limited number of charge-discharge cycles. The efficiency of the electrochemical fuel cell increases with temperature for the practical temperature ranges. Typical fuel cells may have electric outputs ranging from under 1 kW up to megawatts.
Schematically, a fuel cell may be described as a multi-layer system: fuel/current collector/anode/electrolyte/cathode/current collector/oxidizer. A typical solid oxide fuel cell operating on hydrogen may be described as hydrogen/nickel cermet/yttria stabilized zirconia/lanthanum strontium manganite/air. Current collectors are embedded in the anode and cathode.
A major disadvantage of conventional fuel cell design is that the electrode reactions proceed using an inefficient three-phase boundary. To elaborate, the fuel cell electron flow is generated by an electrochemical reaction of fuel oxidation with release of electrons. Conventional oxidation reactions proceed at a three-phase boundary: electrode-electrolyte-gaseous reactant. The actual working surface of the electrodes in this case is very small and does not exceed 1-4% of the apparent electrode surface. Accordingly, more than 95% of the electrode area does not participate in the electricity generation process. Multiple attempts have been made to increase the useful area of the electrodes by introducing mixed (electronic and ionic) conductors into the three-phase boundary. When this is done, working area may increase up to 5-10%. Still, at best, about 90% of the electrode area is not being used.
Fuel cell developers devote major attention to cells operating on gaseous fuel (hydrogen, natural gas, CO). Cells operating on a solid fuel, such as carbon-containing materials (coal, biomass, or waste—both municipal and from the petrochemical industry) have received much less attention. At the same time, operation on solid fuel may have such advantages as: much safer operation (fuel is not flammable or explosive), easier transportation, generally low cost, high power density, and, in some cases such as when carbon-containing fuel is used, much higher efficiency of energy conversion. The last is a result of the near-zero entropy loss in complete electrochemical oxidation of carbon. This translates to efficiency above 70%, while the efficiency of gas-fueled cells is in the 30-50% range.
Use of carbon-containing fuel for electricity generation in electrochemical fuel cells (Direct Carbon Fuel Cell or DCFC) opens an opportunity to eliminate release of fuel oxidation products and contaminants into atmosphere, which is the major problem associated with coal combustion power plants.
For general background on fuel cells, please refer to James Larminie & Andrew Dicks, Fuel Cell Systems Explained (Wiley 2d ed. 2003), and to EG&G Services et al., Fuel Cell Handbook (U.S. Department of Energy, 5th ed. 2000).
A variety of schemes have been proposed for a direct carbon fuel cell. None have as yet come to commercial fruition. For example, U.S. Pat. No. 5,298,340 to Cocks and LaViers stated that “[t]hermodynamic factors favor a solid carbon fuel cell over other fuel cell designs.” They proposed “the dissolution of carbon into a solvent” which would “act[ ] as an anode.” In their subsequent U.S. Pat. No. 5,348,812, Cocks and LaViers taught that “[f]uel cells containing an anode of molten metal into which carbon has been dissolved, and a carbon-ion electrolyte, can be improved by making the molten metal the same as that used as the cation on the solid carbon-ion-electrolyte.”
U.S. Pat. No. 6,607,853 to Hemmes discusses fuel cells based on the oxidation of carbon and carbon-containing materials contained in a molten corrosive salt. The possibility of internal reformation is included, but not explicitly required. In Hemmes' disclosure the molten corrosive salt contacts both the solid electrolyte and the anode. Hemmes discloses that the anode may be porous, made of nickel, and in contact with the solid electrolyte. Hemmes also discloses that the anode may be positioned at a distance from the solid electrolyte.
U.S. Pat. No. 6,692,861 to Tao addresses a fuel cell with a carbon-containing anode and an electrolyte having a melting temperature of between about 300° C. and about 2000° C. in contact with the anode. U.S. Pat. No. 6,200,697 to P. Pesavento of SARA, Inc., Cypress, Calif., describes a concept for generating electricity using a carbon-containing consumable anode. Because that concept employs a consumable anode, it is in essence a large nonrechargeable battery. Subsequently, J. Cooper of Lawrence Livermore National Laboratory has sought to develop a fuel cell employing carbon nanopowder as fuel using molten carbonate electrolyte, similar to the concept developed by Robert D. Weaver at SRI International in the 1970s.
There is thus a need in the art for a direct carbon fuel cell which can be scaled up effectively to a commercially viable size, at a minimum to the hundreds of kilowatts of present-day commercial phosphoric acid cogeneration fuel cell plants or molten carbonate fuel cells, and which can operate efficiently with naturally available fuels such as coal, coke, tar, biomass, and various forms of carbon-containing wastes.